1. Field of the Invention
The present invention relates to substrate processing apparatuses used for processing a substrate such as a wafer. Substrate processing apparatuses are suitable for use as projection exposure apparatuses in which an exposure process for a wafer is performed step-by-step by projecting a reticle pattern onto the wafer at a reduced size.
2. Description of the Related Art
Various methods for aligning a mask, such as a reticle, and a substrate to be exposed, such as a wafer, in a projection exposure apparatus, have been suggested. Generally, two typical aligning methods are known in the art: off-axis global alignment (hereinafter abbreviated as OAAA), and through-the-lens die-by-die alignment (hereinafter abbreviated as TTLAA).
In OAAA, the positions of a plurality of alignment marks formed on a single wafer are determined by an optical system other than a projection optical system. The wafer is moved in steps of a “predetermined distance” by using the alignment marks as a reference, and the areas corresponding to chips are exposed step-by-step. The “predetermined distance” is the interval between the chips on the wafer obtained on the basis of the alignment marks, and it is determined in advance in accordance with the pattern to be formed. Normally, a first shot position relative to the alignment marks, a second shot position relative to the first shot position, a third shot position relative to the second shot position, and so on, are determined in advance, and the interval between the shots is determined to be the “predetermined distance”. In OAAA, the number of times the alignment is performed for a single wafer is small, so the alignment time is short and the exposure process of the wafer can be quickly performed. However, in a semiconductor manufacturing process, the positions of the chips relative to the alignment marks and intervals between the chips may be changed on the submicron scale. In such a case, the positional errors cannot be corrected in OAAA. In contrast, in TTLAA, the reticle and the wafer are aligned before each shot by using the projection optical system. Thus, the above-described disadvantage of the OAAA can be overcome by the TTLAA. However, since the alignment is performed before each shot, the alignment time is long and throughput of the exposure apparatus is reduced.
In order to solve the above-described problems regarding the aligning methods, a substrate processing apparatus has been suggested in, for example, U.S. Pat. No. 5,715,064. This substrate processing apparatus will be described below with reference to FIG. 1. In FIG. 1, reference numeral 111 denotes an exposure processing unit, and 121 denotes a substrate measurement unit in which alignment is performed. In the substrate measurement unit 121, reference numeral 101 denotes an off-axis alignment optical system, 102a denotes a wafer, and 103a denotes a wafer chuck, which retains the wafer 102a by air suction. In the exposure processing unit 111, reference numeral 106 denotes a projection exposure system which projects the pattern of a reticle 106 onto a wafer 102b, 107 denotes a TTL alignment optical system, and 108 denotes an illumination optical system. In addition, reference numeral 109 denotes a central processing unit and 110 denotes hoses used for air suction. In the substrate measurement unit 121, the above-described “predetermined distance”, that is, the distance, by which the wafer 102a placed on the wafer chuck 103a is moved before each shot, is measured. After the measurement, the wafer 102a is transferred to the exposure processing unit 111. The substrate processing apparatus includes at least two identical stages 104a and 104b. The stages 104a and 104b support the wafer chucks 103a and 103b, respectively, and move between the substrate measurement unit 121 and the exposure processing unit 111. The examples of the known art disclose a method for measuring the positions of the stages 104a and 104b. More specifically, as shown in FIG. 1, two interferometers 104a and 104b are disposed in a direction in which the substrate measurement unit 121 and the exposure processing unit 111 are aligned.
With regard to the above-described example of the known art, the inventors have recognized the necessity for considering different situations and dealing with each situation individually.
(1) A case is considered in which a series of processes including wafer supply, measurement, exposure, and wafer removal are performed, while a plurality of stages are being rotated between the substrate measurement unit and the exposure processing unit. In this case, a situation may occur in which laser beams cannot be radiated on the stages while the stages are aligned in a direction perpendicular to the direction in which the substrate measurement unit and the exposure processing unit are aligned (that is, the direction perpendicular to the page). If such a situation occurs, it becomes difficult to measure the positions of the stages while the stages are passing each other.
(2) The stages are provided with hoses for air suction and wires for sensors. Thus, when the stages are rotated between the substrate measurement unit while dragging the hoses and wires, a situation may occur in which the hoses and wires become tangled.